Recent research has shown that inherently nonlinear mechanisms are required to explain many aspects of texture discrimination and motion perception. These mechanisms all incorporate oriented filtering followed by rectification and then a second oriented filtering stage and are termed "non-Fourier" or "second order" processes. The proposed research will test the hypothesis that higher level form vision likewise utilizes non-Fourier shape mechanisms. In this proposal a non-Fourier mechanism capable of processing quasi-circular and ellipsoidal contours is developed, and networks of these units are shown to be capable of analyzing the shapes of curved objects. These non- Fourier form mechanisms also agree with characteristics of concentric units found in cortical area V4 in primates (Gallant et al, 1993) and with V4 functional anatomy (Schoups et al, 1995). Pilot data involving discrimination of circles and Glass (1969) patterns support the existence of these mechanisms in human vision by showing that information is summed linearly along curved contours. Major goals of this proposal are therefore: (1) measure the characteristics of non-Fourier form mechanisms psychophysically; (2) use these mechanisms to predict Glass (1969) pattern perception; (3) gather psychophysical data and use the same mechanisms to predict discrimination for a very general mathematical class of shapes; (4) extend these results to stereopsis; and (5) determine whether other nonlinear models, including that of Kovacs and Julesz (1993, 1994), might explain the data. The study of non-Fourier form mechanisms offers the promise of a new leap forward in our understanding of higher form vision. The principles learned should lead to insights into the perception of human faces and prosopagnosia, into strabismic amblyopia, and may soon permit us to optimize image proceeding systems for analyzing medical images to detect tumors, etc.